Touchscreen systems are interface systems that allow a user to control one or more devices by touching a touch pad or display screen. These systems sense when a user touches the screen such as with single touches, multiple touches, pinches, swipes and other types of touches. The positions and types of these various touches is determined and interpreted by the touchscreen systems so that they can accurately respond to the user commands and input that the touches represent. There are many different methods of sensing these various touches including capacitive sensing (self and mutual), resistance sensing, sound/light wave sensing and vibration sensing. Both types of capacitive sensing systems essentially work by using one or more sensors coupled to a layer of capacitive material adapted to hold an electrical charge. When a user touches the screen it changes the amount of charge stored on the capacitive material at the point or points of contact. This change in the charge will correspond to a change in the capacitance which is measured by the sensors in the system. When there are multiple sensors, they can be used to determine the location of the touch.
One disadvantage of capacitance systems is that capacitance measured by the sensors will include not only the capacitance caused by a user touching the screen, but also the sensor capacitance of each sensor as well as other parasitic capacitances. Furthermore, this sensor and parasitic capacitances can change from sensor to sensor and from touchscreen panel to touchscreen panel. As a result, although exemplary touch events only add 1 pico-Farad of capacitance to the sensors, the charge transfer schemes/integrators used to measure the capacitance must be able to handle the much greater amount of capacitance that represents the touch capacitance added to the sensor and parasitic capacitances. Previously, the problem has been solved by choosing an integrating capacitor that is large enough to not be saturated by the total capacitance/charge received from the sensors. However, using large integrating capacitors undesirably increases cost and size of components, as well as reducing the gain and the resolution of the measurement system.
Another disadvantage of prior capacitance systems is that even if a large enough integration capacitor is chosen to compensate for the initial capacitance added by the sensor capacitance and the parasitic capacitances, these capacitances can change with environmental changes. For example, with increased humidity the added moisture in the air typically increases these unwanted capacitances which can cause an integration cap that was previously large enough to no longer be able to handle the increased measured capacitance due to the environmental changes. In such a case, the measurement system will no longer be able to tell when a sensor is being touched and the touchscreen system will become inoperable until the environment becomes less humid.